The present invention relates to an improved method of manufacturing spherical alumina particles. More particularly, it relates to a method of manufacturing spherical alumina particles which are in physical strength by an improvement of the stage for ageing spherical alumina hydrogel obtained by the oil-drop method. It also relates to a method of manufacturing spherical alumina particles which are superior in physical stability, particularly thermal stability by an improvement of the ageing stage followed by a calcination stage of spherical alumina hydrogel.
In the case of employing spherical alumina particles as a catalyst or as a catalyst carrier, the performance of said particles is judged not only by their reactivity or activity, selectivity and stability, but also by their physical stability.
When spherical alumina particles are employed as a carrier of the catalyst for purifying exhaust gases, the thermal stability of the spherical alumina particles at an elevated temperature constitutes a particularly indispensable requirement. Spherical alumina particles, when used at an elevated temperature, occasionally develop some volume shrinkage the alumina particles due to structural change thereof, and this shrinkage tends to cause voids to appear in the tightly packed catalyst bed, thereby causing the reactant to channel through the catalyst bed without effectively contacting the catalyst. Besides, in the case of a catalyst packed in a reactor or convertor which is subject to rather frequent vibration, the interparticle contact becomes excessive, resulting in a loss of catalyst due to abrasion. While, in the case where the spherical alumina particles are exposed to an elevated temperature, due to thermal sintering of the alumina particles, a part of the pores thereof becomes blocked and the structural change of the pores brings on a considerable reduction of the surface area. This causes extinction of a portion of the active sites of the catalyst, which causes a lowering of the efficiency of the catalyst. The present invention is intended to provide a method of manufacturing spherical alumina particles suitable for use even under such severe conditions.
Spherical alumina particles are advantageously manufactured by the well-known oil-drop method substantially as described in U.S. Pat. No. 2,620,314. Briefly, the method comprises commingling an acidic alumina hydrosol with a weak base which hydrolyzes to ammonia with increasing temperature and effects a strong buffering action, dispersing the resulting mixture as droplets in an appropriate water-immiscible liquid thereby forming substantially spherical alumina hydrogel particles, ageing the thus obtained particles, washing in water, drying and calcining thereafter.
In the prior art, spherical hydrogel particles are usually aged in a hot oil bath in the first place and they are next aged in an alkaline medium such as ammonium hydroxide solution. The concentration of ammonia in said ammonium hydroxide solution is maintained at a fixed concentration in the range of from about 1 wt.% to about 3 wt.%. However, according to this conventional ageing in an ammonium hydroxide solution, the surfaces of the particles are apt to be cracked and this tendency is conspicuous particularly in the case of particles having a diameter of more than about 2 mm. Not only that, the conventional ageing process has a drawback that, in proportion to the lessening of the apparent bulk density of spherical alumina particles, the surface area thereof decreases. Such undesirable phenomena seem to be attributable to the fact that alumina particles before their pore structure is established, come in contact with an ammonium hydroxide solution having a relatively high concentration.
Referring to the calcining stage for alumina particles after ageing, it is common in the prior art to calcine dried spherical alumina particles at a temperature in the range of from about 425.degree. to about 760.degree. C in an oxidizing atmosphere. However, spherical alumina particles thus calcined are not always satisfactory in thermal stability at elevated temperature and the volume shrinkage thereof and the decrease of surface area are relatively great. To be suitable for use as a catalyst or a catalyst carrier at an elevated temperature, spherical alumina particles are required to have a lesser degree of volume shrinkage and decrease of surface area. These drawbacks are considered ascribable to a change in the pore structure and/or the crystal structure of the spherical alumina particles due to thermal sintering. At present, however, in order to enhance the resistance of spherical alumina particles to elevated temperature, a complicated step such as addition of alkaline earth metal and so on thereto must be resorted to. In a method as above-mentioned, however, it is an inevitable disadvantage that the commercial value of the spherical alumina particles as a catalytic carrier is greatly decreased depending on its use.